The present invention relates to a N-fluoropyridinium salt and a process for preparing same. The N-fluoropyridinium salts according to the present invention are very useful as a fluorine atom introducing agent as seen from the examples 66-133 hereinafter illustrated. The salts according to the invention have a widespread use because of their high reactivity with a wide variety of compounds and selectivity for any desired products. For example, said salt can be used for the preparation of 3-fluoro-4-hydroxyphenlacetic acid which is useful as a thyroid inhibitor by reacting the former with p-hydroxyphenylacetate followed by a common hydrolysis reaction as illustrated in examples 79 to 82 referred to hereinafter.
Heretofore, it has been well known in the art that fluorine compounds are significantly distinguished from chlorine compounds, bromine compounds, and iodine compounds in their physical properties and reactivities, because fluorine atom have characteristics such as very high electronegativity, high ionization energy, extremely high bonding ability with other atoms, small Van der Waals diameter, lack of a d-electron orbit and the like (N. Ishikawa & Y. Kobayashi; FLUORINE COMPOUNDS; THEIR CHEMISTRY AND APPLICATIONS; KodanshaSchientific, pp. 69-70. 1979). Therefore, fluorination reactions naturally have significantly different aspects from other halogenation reactions such as chlorinations, brominations and iodinations.
In reactions with organic compounds, fluorine, contrary to chlorine, bromine and iodine, reacts very violently, readily giving rise to the fission of the C--C bond of organic compounds and in cases where the reaction is excessively violent, fire or explosion in turn can break out. The abnormality of fluorination reactions relative to other halogenation reactions may be readily understood from the comparison of heat of formation in halogenation reactions (see the description on pages 69-75 of the above article) as follows:
______________________________________ .DELTA.H (Kcal/mol) type of reaction X = F Cl Br I ______________________________________ C.dbd.C + X.sub.2 --CX--CX -111 -36 -23 -16 C--H + X.sub.2 --C--X + HX -105 -25 -9 +6 ______________________________________
As seen from the above Table, since the heat of reaction in the fluorination reactions amount to ever 100Kcal/-mol, while the bonding energy between carbon-carbon atoms is approximately only 60Kcal/mol, the control of fluorination reactions is very difficult, contrary to other halogenation reactions. Accordingly, the development of fluorination reactions having better selectivity has been an important subject matter in fluorination industries.
For the purpose resolving the above problem, a wide variety of compoounds for introducing fluorine atoms have heretofore been studied and developed. As such compounds, for example, trifluoromethyl-hypofluorite (CF.sub.3 OF), trifluoroacetyl-hypofluorite (CF.sub.3 COOF), acetyihypofluorite (CH.sub.3 COOF), xenon difluoride XeF.sub.2), FClO.sub.3, sulfur tetrafluoride (SF.sub.4), die-thylaminosulfur trifluoride (ET.sub.2 NSF.sub.3), CClHFCF.sub.2 NEt.sub.2,CF.sub.3 CFHCF.sub.2 NEt.sub.2, heavy metal fluorides such as AgF, HgF, CoF.sub.3,AgF.sub.2 and the like were known in the art (see pages 79-94 of the above-mentioned article). However, these compounds have drawbacks such as poor selectivity for the desired reaction, are highly hazardous to handle, have high cost, unstableness, a limited scope of application, and the like which make them commercially unsatisfactory. On the other hand, hydrogen fluoride, hydrofluoric acid, potassium fluoride, cesium fluoride, and the like which are known as inexpensive agents for introducing fluorine atoms are inferior in electrophilic reactivity, which imposes such limitations that they cannot perform electrophilic substitutions for aromatic nuclei or negatively charged carbon ions. These compounds also present serious problems in handling because hydrogen fluoride or hydrofluoric acid, for example, are highly toxic. It has been suggested that a pyridine. F.sub.2 complex can be used as a fluorine atom-introducing agent, but it can only offer low total yield of fluorination reactions [see, Z. Chem., 12, 292 (1972)] and moreover, said complex is highly hygroscopic and thermally unstable so that explosions may break out at above -2.degree.C. [Z. Chem., 5, 64 (1965)]. From the above, it can hardly be said that the complex is a useful fluorinating agent. Recently, N-fluoro-N-alkylarenesulfoneamide have been reported as fluorine atom-introducing agents, but these compounds are low in reactivity and only effective for particular reaction species (negatively charged carbon ions) [J. Amer. Chem. Soc. 106, 452 (1984)]. Therefore, a strong need exists for the development of highly satisfactory fluorine atom-introducing agents.
As a result of a series of earnest investigations by the present inventors towards the development of a novel fluorine-introducing agent, they have succeeded in developing a novel fluorine-introducing agent which is active but stable allowing the easy handling of the agent which still retains high selectivity of a desired reaction, thus completing the present invention. The compounds according to the present invention have high reactivity with a variety of compounds and high selectivity for any desired compounds, which allows the compounds to be very useful for the synthesis of a variety of fluorine-containing compounds in a shortened process. For example, a thyroid inhibitor, 3-fluoro4-hydroxy-phenylacetic acid could easily be prepared from p-hydorxyphenylacetate available industrially (see, Examples 79-82 hereinafter described).